A free Shakespeare production on the Boston Common has become quite the summer tradition, thanks to Commonwealth Shakespeare Company.This year the show is "Twelfth Night," one of the Bard's more popular romantic comedies, featuring identical twins, a sister and brother who are separated when their boat is shipwrecked on the coast of Illyria, and they each fear the other is drowned.Shakespeare's text begins with the sister, Viola, having landed on shore. But director Steven Maler starts the show with an extended pantomime of the storm with sails billowing and the sailors and passengers being tossed into the sea and struggling to survive. A kind of dance between Viola (Marianna Bassham) and her brother Sebastian (Nile Hawver) indicate how close they are. It's a lovely, creative opening.Once Viola is on shore, she confers with a loveable Captain (Jerry Goodwin), who says he saw Sebastian tie himself to a mast and feels there is hope he may have survived.In a nice scene shift, Orsino (Robert Najarian), who governs Illyria, comes on stage to talk with his aides. When they're done and start to leave, the shipwrecked sailors rise from the stage where they had been lying. You may have forgotten they were there. It's as if they share the same space without realizing the other group is there.Viola takes on the character of a man, calling herself Cesario, and dresses in a turquoise stocking cap and a turquoise patterned shirt. She soon falls in love with her employer, Orsino, but can't reveal her love for him. After all, he thinks she's a boy. Najarian plays Orsino with an intense passion for Olivia, the daughter of a count, who shows no interest in him. But he also expresses interest in Viola without realizing he's doing it. It's a lovely, delicate thing. At one point, he leans his head against Viola, causing her to fall back and lie down with his head ending up in her lap. It's a charming moment.Kerry O'Malley gives an exquisite performance as Olivia, shifting fluidly from one emotional position to another. When Viola as Cesario arrives at her house to plead for Orsino, Olivia wants nothing to do with any of it. They have a lively, spunky fight between them. But suddenly, Olivia shifts as if on a dime, and she's clearly falling in love with Cesario, a love that grows with intensity when Cesario next returns.Bassham has her own liveliness and variety as Viola/Cesario, but it's a little too broad and hard to see how Olivia would fall in love with "him."As in all of Shakespeare's comedies, there's a group of low-life comic characters. This group is headed by Sir Toby Belch, who's played by Robert Pemberton with considerable energy and charm and ends up being drunk much of the time. Conner Christiansen plays his buddy Sir Andrew Aguecheek as a very effeminate gay with lots of stylized mannerisms. Sir Andrew has a pointed Mohawk hairdo and first greets Sir Toby by jumping up and throwing his legs around his waist. The two have a lot of good-natured fun.Sheree Galpert brings considerable spark to Maria, Olivia's employee. She helps plan a letter that she, Sir Toby, and Sir Andrew leave for the stuffed shirt Malvolio (Fred Sullivan Jr.) to come across. He assumes that Olivia has written to him, having fallen in love with him. He's very funny trying to pronounce a few letters that don't quite add up to his name and then appearing, as the letter suggests, in cross-gartered yellow stockings, assuming this is how Olivia most wants to see himIn the second act, Sebastian appears, causing people to think he is his twin sister Viola/Cesario, with some funny and life-changing consequences.Woody Gaul brings appealing intensity and seriousness to Antonio, who finds Sebastian when he first washes up on shore and looks out for him. Juan Rodriguez has an eager freshness as Fabian who joins the band of comic characters. And Remo Airaldi gives a poignant touch to Feste, Olivia's fool, who has the last speech of the play.Cristina Todesco's set is based on a psychedelic mural created by Victor Reyes in Wynwood Walls, a warehouse district covered with street art, in Miami.The play is based on identity confusion that's a bit improbable, but if you are willing to go with it, there's plenty of fun to be had."Twelfth Night"WHEN: Through Aug. 10WHERE: Boston Common, near the Parkman BandstandCOST: FreeINFO: 617-426-0863; www.commshakes.org